[PATCH] vmap(): don't allow invalid pages

[Russell King (Oracle)]

On Thu, Jan 20, 2022 at 12:22:35PM +0000, Robin Murphy wrote:
> On 2022-01-19 19:12, Russell King (Oracle) wrote:
> > On Wed, Jan 19, 2022 at 06:43:10PM +0000, Robin Murphy wrote:
> > > Indeed, my impression is that the only legitimate way to get hold of a page
> > > pointer without assumed provenance is via pfn_to_page(), which is where
> > > pfn_valid() comes in. Thus pfn_valid(page_to_pfn()) really *should* be a
> > > tautology.
> > 
> > That can only be true if pfn == page_to_pfn(pfn_to_page(pfn)) for all
> > values of pfn.
> > 
> > Given how pfn_to_page() is defined in the sparsemem case:
> > 
> > #define __pfn_to_page(pfn)                              \
> > ({	unsigned long __pfn = (pfn);                    \
> > 	struct mem_section *__sec = __pfn_to_section(__pfn);    \
> > 	__section_mem_map_addr(__sec) + __pfn;          \
> > })
> > #define page_to_pfn __page_to_pfn
> > 
> > that isn't the case, especially when looking at page_to_pfn():
> > 
> > #define __page_to_pfn(pg)                                       \
> > ({      const struct page *__pg = (pg);                         \
> >          int __sec = page_to_section(__pg);                      \
> > 	(unsigned long)(__pg - __section_mem_map_addr(__nr_to_section(__sec))); \
> > })
> > 
> > Where:
> > 
> > static inline unsigned long page_to_section(const struct page *page)
> > {
> > 	return (page->flags >> SECTIONS_PGSHIFT) & SECTIONS_MASK;
> > }
> > 
> > So if page_to_section() returns something that is, e.g. zero for an
> > invalid page in a non-zero section, you're not going to end up with
> > the right pfn from page_to_pfn().
> 
> Right, I emphasised "should" in an attempt to imply "in the absence of
> serious bugs that have further-reaching consequences anyway".
> 
> > As I've said now a couple of times, trying to determine of a struct
> > page pointer is valid is the wrong question to be asking.
> 
> And doing so in one single place, on the justification of avoiding an
> incredibly niche symptom, is even more so. Not to mention that an address
> size fault is one of the best possible outcomes anyway, vs. the untold
> damage that may stem from accesses actually going through to random parts of
> the physical memory map.

I don't see it as a "niche" symptom.

If we start off with the struct page being invalid, then the result of
page_to_pfn() can not be relied upon to produce something that is
meaningful - which is exactly why the vmap() issue arises.

With a pfn_valid() check, we at least know that the PFN points at
memory. However, that memory could be _anything_ in the system - it
could be the kernel image, and it could give userspace access to
change kernel code.

So, while it is useful to do a pfn_valid() check in vmap(), as I said
to willy, this must _not_ be the primary check. It should IMHO use
WARN_ON() to make it blatently obvious that it should be something we
expect _not_ to trigger under normal circumstances, but is there to
catch programming errors elsewhere.

-- 
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